Transformable Lightweight Structural Steel

ABSTRACT

A transformable lightweight structural steel, which exhibits a resistance to hydrogen embrittlement, has TRIP and TWIP properties and contains the following elements (in wt.-%): C 0.05 to &lt;=1.0; Al 0.0 to &lt;=11.0; Si 0.0 to &lt;=6.0; Al+Si&gt;0.05; Mn 9.0 to =25.0; H&lt;20 ppm, the remainder being iron including incidental steel companion elements, whereby different phases are present depending on the alloy composition. The lightweight structural steel is characterized by associating a higher C content with a lower Mn content and associating a low C content with a higher Mn content, with the C—Mn value pairs being positioned in a C—Mn coordinate system approximately on a straight connecting line that is distant from the connecting line of the C—Mn value pairs being in balance between the austenite and martensite phases.

The invention relates to a transformable lightweight structural steel with TRIP (Transformation Induced Plasticity) and TWIP (Twinning Induced Plasticity) characteristics according to the preamble of claim 1.

Transformable lightweight structural steels are known (DE 10 2004 061 284 A1, DE 197 27 759 A1, DE 101 285 44 A1). The presence of residual stress in the material may lead in these and comparable steels in dependence on the structure and strength to a lagging embrittlement which is triggered by hydrogen and ultimately to cracking.

To overcome this problem, it has been proposed to limit the hydrogen content to <20 ppm, preferably to <5 ppm (DE 10 2004 061 284 A1).

This proposal, though helpful, is inadequate because even when setting low hydrogen contents the effect of hydrogen embrittlement can still occur. Moreover, the set maximum value for hydrogen may still be exceeded during steel production for various reasons, a fact that can be tolerated during alloying but increases the risk of encountering hydrogen embrittlement.

It is an object of the invention to provide a lightweight structural steel of a type involved here to have very good mechanical properties (ductility, strength) in the absence of a lagging hydrogen embrittlement.

Based on the preamble, this object is solved in combination with the characterizing features of claim 1. Advantageous improvements are the subject matter of subclaims.

According to the teaching of the invention, the problem stated in the formulation of the object is solved by a new alloying concept. This is characterized by associating a higher C content to a lower Mn content and a lower C content to a higher Mn content, with the C—Mn value pairs lying approximately in a C—Mn coordinate system on a straight connection line which is distant to the connection line of C—Mn value pairs that are in balance between γ (austenite fcc) and α′ phases (martensite bcc).

This novel alloying concept is cognizant of the fact that the γ austenite(fcc) and the ε martensite(hcp) phase have a high hydrogen solubility while the α′ martensite(bcc) phase has a significantly smaller hydrogen solubility. In the presence of the TRIP effect, the α′martensite phase is formed depending on the alloying composition, partly via the metastable ε martensite phase. In regions where the material is transformed, e.g. through pressure stress, the more densely packed ε martensite phase may be present even after transformation according to the principle of least restraint and convert to the α′martensite phase when relieved.

The conversion from the ε martensite phase to the α′martensite phase causes the hydrogen to escape as a result of the lower solubility and leads either atomically or recombinant to a material weakening, possibly to cracking.

Based on an alloy with C and Mn, the addition of Al and/or Si results in a destabilization of the ε martensite phase. This reduces the risk of hydrogen embrittlement and increases the leeway for the steel worker to classify the poured melt as still tolerable even when the maximum value of hydrogen is exceeded. Less devaluation increases the yield and thus the cost-effectiveness of the process.

Preferably, the addition of Al and Si is substantially the same.

Regardless of the effect of the addition of Al and/or Si, the carbon content is a crucial element in the proposed alloying concept as it stabilizes the austenite phase and displaces hydrogen from the free lattice sites.

The scatter band about the connection line of optimum C—Mn value pairs for the content of C should amount to =±0.15%, preferably ±0.1%, for the content of Mn=±2.5%, preferably ±1.5%.

For example, alloys with

0.7% C, 15% Mn, 2.5% Al, 2.5% Si

as well as

0.4% C, 18% Mn, 2.5% Al, 2.5% Si

exhibit, as described hereinafter, no lagging crack formation (“delayed fracture”) besides superior mechanical properties.

After annealing at 850° C., the first alloy example has a yield point R_(p0.2) of 480 MPa and a strength of 850 MPa with an elongation A of 58%. These values for the second alloy example also after annealing at 850° C. are R_(p0.2) of 450 MPa, R_(m) of 790 MPa and A of 53%. A second characteristic quantity is the product of strength×elongation, which is a measure for the performance of the material. This value is at 49,300 for the alloy example 1 and at 41,870 (%×MPa) for example 2.

The sole FIGURE shows the C content as a function of the Mn content as plotted in a coordinate system. The continuous straight connection line shows the C—Mn value pairs in balance with respect to the γ austenite and the α′ martensite phases, with consideration of addition of Al and/or Si.

The dashed connection line which is distant to the balance line characterizes value pairs of the optimum alloying concept with respect to material properties in the absence of a lagging crack formation (delayed fracture). The hatching shown across the dashed line is intended to indicate the qualitative scatter band within which optimum properties can still be expected. 

1.-4. (canceled)
 5. A transformable lightweight structural steel with TRIP and TWIP characteristics, comprising the elements, in weight-%, C 0.05 to ≦1.0 Al 0.0 to ≦11.0 Si 0.0 to ≦6.0 Al+Si>0.05 Mn 9.0 to ≦25.0 H<20 ppm, remainder iron including incidental steel elements, wherein different phases may be present in dependence on the alloy composition, wherein a higher C content is associated to a lower Mn content and a lower C content is associated to a higher Mn content, with C—Mn value pairs 0.7C/15Mn and 0.4C/18Mn lying approximately in a C—Mn coordinate system on a straight connection line which is distant to a connection line of C—Mn value pairs in balance between γ (austenite) and α′phases (martensite).
 6. The lightweight structural steel of claim 5, wherein the content of Al and Si is substantially the same.
 7. The lightweight structural steel of claim 5, wherein the C content has a tolerance range of ±0.15% and the Mn content has a tolerance range of ±2.5% in relation to the straight connection line.
 8. The lightweight structural steel of claim 5, wherein the C content has a tolerance range of ±0.1% and the Mn content has a tolerance range of ±1.5% in relation to the straight connection line. 